1. Field of the Invention
This invention relates to electronic devices that include a layer of graphene.
2. Discussion of the Related Art
In a recent article in Physics Today M. Wilson reviews a unique two-dimensional condensed matter system known as graphene, a single, one-atom thick sheet (i.e., a monolayer) of carbon atoms arranged in a honeycomb, hexagonal lattice. Graphene is the building block for graphite-like carbon materials of every other dimensionality. [See, “Electrons in Atomically Thin Carbon Sheets Behave like Massless Particles,” Phys. Today, p. 21 (January 2006), which is incorporated herein by reference.]
Wilson points out that graphene has rather remarkable properties. First, it is stable, chemically inert, and crystalline under ambient conditions. Second, it is a semimetal in that its conduction and valence bands just meet at discrete points in the Brillouin zone. Third, an electron in graphene has an effective mass of zero and behaves more like a photon than a conventional massive particle. Fourth, it can carry huge current densities—about 108 A/cm2, roughly two orders of magnitude greater than copper.
In the last few years scientists have attempted to create a single 2D graphene sheet in a free state. For example, one group used adhesive tape to peel off weakly bound layers from a graphite crystal, gently rubbed those fresh layers against an oxidized silicon surface, and then identified the relatively few monolayer flakes among the macroscopic shavings. [See, for example, K. S. Novoselov et al., Science, Vol. 306, p. 666 (2004), which is incorporated herein by reference. Another group fabricated ultrathin carbon films, typically three graphene sheets, by thermal decomposition of the surface of SiC. The SiC was simply heated sufficiently to evaporate Si from the surface, leaving behind the thin carbon films. [See, C. Berger et al., J Phys. Chem. B, Vol. 108, p. 19912 (2004), which is incorporated herein by reference.]
Growing single sheets of graphene, however, remains a problem, according to Wilson the monolayers of carbon are most vulnerable to damage when being heated during typical fabrication processes.
In addition, although Berger et al. refer to their carbon films as having been “grown epitaxially” on SiC, their process actually involved only evaporation of Si; it did not involve “growth” as that term is conventionally used to describe epitaxial deposition in standard fabrication techniques, such as Molecular Beam Epitaxy (MBE) and Chemical Vapor Deposition (CVD). In fact, SiC and graphite have a relatively large lattice-mismatch of about 20%, which is typically much too large for depositing device-quality epitaxial graphite layers on a SiC substrate.
In a preliminary demonstration of device potential, Berger et al. reported that the resistance modulation of a large-area gated graphitic channel FET structure at cryogenic temperatures (4 K) was rather small (only 2%) because the gate covered only a portion of the graphite film between the source and drain electrodes, leaving large ungated leakage paths.
The Berger et al. device is also described by W. A. DeHeer et al. in US Patent Application No. 2005/0253820 published on Dec. 16, 2004.
Thus, a need remains in the art for graphene-based devices in which graphene is epitaxially deposited on a suitable, lattice-matched substrate.